It depends on the TLD. For example for my national domain, I can just send a request form with my signature to the national TLD registry operator, and that's it

It depends on the TLD. For example for my national domain, I can just send a request form with my signature to the national TLD registry operator, and that's it. My domain becomes untransferable, and I also have an option to disable change of NS set. It's free, except for the official signature verification, that can be done at the post office for $1.5.